MINOR: filters: Use a proxy flag to detect use of legacy 'filter' directive

MINOR: filters: Use a proxy flag to detect use of legacy 'filter' directive

Replace the FLT_CFG_FL_LEGACY flag set on each filter configuration with
the PR_FL_FILTER_LEGACY proxy flag, set when an explicit legacy "filter"
directive is parsed.

The legacy or instance mode is a property of the proxy, not of an
individual filter configuration, so flt_check_config_mode() no longer
needs to walk px->filter_configs to detect a legacy filter. Add the
inline helper flt_use_legacy_filter() to expose the proxy flag and use it
in flt_check_config_mode().

This prepares selecting the legacy filter configurations or the filter
instances on a per-proxy basis.